1. Field of the Invention
The present invention relates to an image-forming apparatus of an electrophotographic system and a developing device used for such an image-forming apparatus.
2. Description of the Related Art
With respect to developing systems used for the image-forming apparatus of an electrophotographic system, a mono-component developing system using only toner as a main component of a developer and a two-component developing system using a toner and a carrier as main components of a developer have been known.
A developing device of the mono-component developing system is provided with a toner supporting member that supports toner thereon and transports the toner and a toner-regulating blade that is made contact with the toner supporting face of the toner supporting member. Upon passing through the contact position with the toner-regulating blade, the toner, supported on the toner supporting member, is made friction-contact with the toner-regulating blade to be formed into a thin film, and also charged to a predetermined polarity. In this manner, since the mono-component developing device carries out a toner-charging process through the friction-contact with the toner-regulating blade, the resulting advantage is that the structure is simple, small, and inexpensive. Moreover, since only the toner layer having an upper limit value of 10+several micron meters in thickness is formed on the toner supporting member, the toner supporting member and the photosensitive member can be designed with a fine gap maintained therebetween, and consequently, a high toner transporting speed can be achieved by setting a strong electric-field so that high image quality with high precision can be obtained. However, toner degradation tends to occur due to a high stress in the regulating portion, and the quantity of charge of toner tends to be lowered in endurance use. Moreover, the charge-applying property to the toner is lowered by contamination of the toner-regulating blade and the surface of the toner supporting member due to the toner and external additives to cause problems of fogging and the like, with the result that it becomes difficult to prolong the service life of the developing device.
Since the developing device of the two-component developing system charges the toner and the carrier to predetermined polarities by making them in friction-contact with each other, the stress to be applied to the toner is smaller in comparison with that of the mono-component developing device. Since the surface area of the carrier is larger in comparison with that of the toner, the carrier is less vulnerable to contamination due to adhesion of the toner. However, the length of a magnetic brush formed on a developer supporting member by the carrier is 20 to 50 times thicker than the thickness of the toner layer on the toner supporting member of the mono-component developing system, and the magnetic brush becomes uneven from the microscopic viewpoint. As a result, inevitably, a weaker electric field needs to be set in comparison with that of the mono-component developing system by taking into consideration prevention of leak or the like, and at least one portion of the magnetic brush needs to be made in contact with the photosensitive member. Consequently, since the toner transporting speed becomes slower, and since scraping off of the toner image on the photosensitive member by the carrier occurs, the image quality becomes inferior to that of the mono-component developing system.
As a developing system that adopts the advantages of the two developing systems, a hybrid developing system has been proposed in which an electric field is formed between a transporting roller (first transporting member) on which a developer charged by the two-component developing system is held and a developing roller (second transporting member), and only the toner is consequently separated to form a toner layer on the developing roller so that a mono-component developing process is carried out (Japanese Patent-Application Laid-Open No. 2003-15380). In this developing system, at an opposing portion between the developing roller and the transporting roller, only the toner is selectively supplied from the transporting roller to the developing roller, and residual toner is recovered from the developing roller to the transporting roller. This system makes it possible to achieve both of the long service life of the apparatus and high image quality. In this system, however, at the opposing portion between the developing roller and the transporting roller, the toner supplying and recovering processes become insufficient to cause a new problem in that the toner layer on the developing roller is not sufficiently reset. For this reason, an image memory occurs due to differences in the toner amounts per unit area between the new and previous toner layers on the developing roller as well as in the quantities of charge in the toner. This problem is caused by difficulty in achieving the two directly-opposed functions that, while a new toner is being supplied from the transporting roller to the developing roller, residual toner on the developing roller after the developing process has to be recovered by the developer on the transporting roller, at an area close to the transporting roller and the developing roller. In the case when the toner supply is preferentially set so as to provide high quality images, high speed and the like of the apparatus, the occurrence of image memory becomes particularly conspicuous. For example, in an attempt to meet the recent trend of high charge quantity per unit mass so as to achieve a small size of the toner and also meet demands in the market for high quality image, when the adhering strength (mirror image strength) to the developing roller is increased, or when the toner amount of supply to the developing area per unit time is increased so as to meet the demands for high speed, the recovering property is lowered, with the result that the occurrence of image memory tends to become conspicuous. In contrast, when the toner supply is insufficient, the image density consequently becomes insufficient.
The present invention is to provide a hybrid developing device and an image-forming apparatus that can produce images in which a sufficient image density is achieved and the occurrence of image memory is sufficiently prevented, for a long period.